Re: [PATCH] aarch64: disable LDP via tuning structure for -mcpu=ampere1

[Philipp Tomsich <philipp.tomsich@vrull.eu>]

Kyrylo,

On Fri, 14 Apr 2023 at 11:21, Kyrylo Tkachov <Kyrylo.Tkachov@arm.com> wrote:
>
> Hi Philipp,
>
> > -----Original Message-----
> > From: Philipp Tomsich <philipp.tomsich@vrull.eu>
> > Sent: Friday, April 14, 2023 12:22 AM
> > To: gcc-patches@gcc.gnu.org
> > Cc: Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>; Philipp Tomsich
> > <philipp.tomsich@vrull.eu>; Di Zhao <di.zhao@amperecomputing.com>
> > Subject: [PATCH] aarch64: disable LDP via tuning structure for -
> > mcpu=ampere1
> >
> > AmpereOne (-mcpu=ampere1) breaks LDP instructions into two uops.
> > Given the chance that this causes instructions to slip into the next
> > decoding cycle and the additional overheads when handling
> > cacheline-crossing LDP instructions, we disable the generation of LDP
> > isntructions through the tuning structure from instruction combining
> > (such as in peephole2).
> >
> > Given the code-density benefits in builtins and prologue/epilogue
> > expansion, we allow LDPs there.
>
> LDPs are indeed quite an important part of the ISA for code density and there are, in principle, second-order benefits from using them, like keeping the instruction cache footprint low (which can be important for large workloads).
> Did you gather some benchmarks showing a benefit of disabling them in this manner?


This has been benchmark-driven, but I need to follow up separately (as
I the final numbers are with the folks that have access to the
benchmark machines)..

>
> > This commit:
> >  * adds a new tuning option AARCH64_EXTRA_TUNE_NO_LDP_COMBINE
> >  * allows -moverride=tune=... to override this
> >
> > Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
> > Co-Authored-By: Di Zhao <di.zhao@amperecomputing.com>
> >
> > gcc/ChangeLog:
> >
> >       * config/aarch64/aarch64-tuning-flags.def
> > (AARCH64_EXTRA_TUNING_OPTION):
> >       Add AARCH64_EXTRA_TUNE_NO_LDP_COMBINE.
> >       * config/aarch64/aarch64.cc (aarch64_operands_ok_for_ldpstp):
> >       Check for the above tuning option when processing loads.
> >
> > ---
> >
> >  gcc/config/aarch64/aarch64-tuning-flags.def | 3 +++
> >  gcc/config/aarch64/aarch64.cc               | 8 +++++++-
> >  2 files changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/gcc/config/aarch64/aarch64-tuning-flags.def
> > b/gcc/config/aarch64/aarch64-tuning-flags.def
> > index 712895a5263..52112ba7c48 100644
> > --- a/gcc/config/aarch64/aarch64-tuning-flags.def
> > +++ b/gcc/config/aarch64/aarch64-tuning-flags.def
> > @@ -44,6 +44,9 @@ AARCH64_EXTRA_TUNING_OPTION
> > ("cheap_shift_extend", CHEAP_SHIFT_EXTEND)
> >  /* Disallow load/store pair instructions on Q-registers.  */
> >  AARCH64_EXTRA_TUNING_OPTION ("no_ldp_stp_qregs",
> > NO_LDP_STP_QREGS)
> >
> > +/* Disallow load-pair instructions to be formed in combine/peephole.  */
> > +AARCH64_EXTRA_TUNING_OPTION ("no_ldp_combine",
> > NO_LDP_COMBINE)
> > +
> >  AARCH64_EXTRA_TUNING_OPTION ("rename_load_regs",
> > RENAME_LOAD_REGS)
> >
> >  AARCH64_EXTRA_TUNING_OPTION ("cse_sve_vl_constants",
> > CSE_SVE_VL_CONSTANTS)
> > diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
> > index f4ef22ce02f..8dc1a9ceb17 100644
> > --- a/gcc/config/aarch64/aarch64.cc
> > +++ b/gcc/config/aarch64/aarch64.cc
> > @@ -1971,7 +1971,7 @@ static const struct tune_params ampere1a_tunings
> > =
> >    2, /* min_div_recip_mul_df.  */
> >    0, /* max_case_values.  */
> >    tune_params::AUTOPREFETCHER_WEAK,  /* autoprefetcher_model.  */
> > -  (AARCH64_EXTRA_TUNE_NONE),         /* tune_flags.  */
> > +  (AARCH64_EXTRA_TUNE_NO_LDP_COMBINE),       /* tune_flags.  */
> >    &ampere1_prefetch_tune
> >  };
> >
> > @@ -26053,6 +26053,12 @@ aarch64_operands_ok_for_ldpstp (rtx
> > *operands, bool load,
> >    enum reg_class rclass_1, rclass_2;
> >    rtx mem_1, mem_2, reg_1, reg_2;
> >
> > +  /* Allow the tuning structure to disable LDP instruction formation
> > +     from combining instructions (e.g., in peephole2).  */
> > +  if (load && (aarch64_tune_params.extra_tuning_flags
> > +            & AARCH64_EXTRA_TUNE_NO_LDP_COMBINE))
> > +    return false;
>
> If we do decide to do this, I think this is not a complete approach. See the similar tuning flag AARCH64_EXTRA_TUNE_NO_LDP_STP_QREGS.
> There's various other places in the backend that would need to be adjusted to avoid bringing loads together for the peephole2s to merge (the sched_fusion stuff).
> Plus there's the cpymem expansions that would generate load pairs too...

I have add-on patches for these, but given that I don't have direct
access to the benchmarking machine and the benchmarks have been run
with this functionality only, I didn't submit them for the time being.
Do you see a path to get this in during the current cycle and defer
the add-on patches (happy to resubmit as a series) only?

> We'd want some testcases added to check that LDPs are blocked too...
>
> Thanks,
> Kyrill
>
> > +
> >    if (load)
> >      {
> >        mem_1 = operands[1];
> > --
> > 2.34.1
>